In conventional flat panel display systems, such as liquid crystal display (LCD) systems, the brightness of each pixel or element is controlled by a transistor. An active matrix display includes a grid of transistors (e.g., thin film transistors) arranged in rows and columns. A column line is coupled to a drain or a source associated with each transistor in each column. A row line is coupled to each gate associated with the transistors in each row. A row of transistors is activated by providing a gate control signal to the row line which turns on each transistor in the row. Each activated transistor in the row then receives an analog voltage value from its column line to cause it to emit a particular amount of light. Generally speaking, a column driver circuit provides the analog voltage to the column lines so that the appropriate amount of light is emitted by each pixel or element. The resolution of a display is related to the number of distinct brightness levels. For a high quality display, a multi-reference voltage buffer (e.g., eight or more voltages) is needed to supply voltages to the column driver.
FIG. 1 shows a simplified block diagram of a flat panel display 2, along with its driver circuitry. The driver circuitry includes a column driver 4, which obtains analog voltages from multi-reference voltage buffer 6. Although FIG. 1 shows the driver circuitry logically separate from the flat panel display 2, commercial flat panel displays typically combine the display and the driver circuitry into a single, thin package. Therefore, a major consideration in developing circuitry for such displays is the microchip die size required to implement such logic.
To achieve multi-reference voltage outputs, digital to analog converters (DAC's) can be used to generate different voltages. Capacitors can be coupled to the DAC's to temporarily buffer the voltages. Such a multi-reference voltage circuit has been conventionally implemented in several ways. One way uses a multi-DAC structure as shown in FIG. 2 wherein one DAC is used to drive a buffer for each channel. DAC circuits are very large, however. With such a multi-DAC structure, as the number of outputs increase the chip die size will become undesirably large. Another way of implementing a multi-reference voltage buffer is to use a resistor or capacitor ladder with a switching network as illustrated in FIG. 3. Because all outputs operate independently, for an N-bit converter with M outputs, M·2N switches are needed. Thus, the number of switches required can occupy an undesirably large space. What is needed is a multi-reference voltage buffer small enough to be used in flat panel display packages.